Magnification changing lens

Abstract

This specification discloses a magnification changing lens which has, in succession from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, a third lens unit of negative refractive power and a fourth lens unit of positive or negative refractive power and in which at least the second and third lens unit are moved toward the object side to thereby accomplish the magnification change from the wide angle end to the telephoto end. This magnification changing lens particularly satisfies the following condition: 0.13<bfxmin/fw<0.7, where bfxmin is the shortest distance of the back focus in the whole magnification change range and fw is the focal length of the entire system at the wide angle end.

What is claimed is: 
     
       1. A magnification changing lens comprising in succession from an object side: a first lens unit having a negative refractive power;   a second lens unit having a positive refractive power;   a third lens unit having a negative refractive power; and   the last lens unit;   wherein said second lens unit and said third lens unit are moved toward the object side during magnification change from wide angle end to telephoto end, and unit spacing between said first lens unit and said second lens unit and unit spacing between said second lens unit and said third lens unit at the telephoto end are smaller and unit spacing between said third lens unit and said last lens unit at the telephoto end is greater than at the wide angle end.   
     
     
       2. A magnification changing lens according to claim 1, wherein the paraxial lateral magnification of said last lens unit always has a positive value. 
     
     
       3. A magnification changing lens according to claim 2, wherein said last lens unit has a positive refractive power. 
     
     
       4. A magnification changing lens according to claim 2 wherein, when an amount of movement of said third lens unit and said last lens unit during the magnification change from the wide angle end to the telephoto end are M3 and M4, respectively, and an amount of movement toward the imaging plane side is positive, the following condition is satisfied:   -1<M4/M3<0.8.     
     
     
       5. A magnification changing lens according to claim 4, whwerein said last lens unit is fixed during magnification change. 
     
     
       6. A magnification changing lens according to claim 4, wherein when the focal lengths of said second and third lens units are f2 and f3, respectively, and the following condition is satisfied:   0.8<|f3/f2|<1.4.     
     
     
       7. A magnification changing lens according to claim 6, wherein when the focal length of said last lens unit is f4 and the focal length of the entire system at the wide angle end is fw, and the following condition is satisfied:   1.5<f4/fw<7.0.     
     
     
       8. A magnification changing lens according to claim 2, wherein said last lens unit has a negative refractive power. 
     
     
       9. A magnification changing lens according to claim 8, wherein said last lens unit always has a magnification of one-to-one or higher. 
     
     
       10. A magnification changing lens according to claim 9, wherein when an amount of movement of said third lens unit and said last lens unit during the magnification change from the wide angle end to the telephoto end are M3 and M4, respectively, following condition is satisfied:   0≦M4/M3<3.     
     
     
       11. A magnification changing lens according to claim 10, wherein said last lens unit is fixed during magnification change. 
     
     
       12. A magnification changing lens according to claim 10, wherein said last lens unit has at least one negative lens having its concave surface facing the object side. 
     
     
       13. A magnification changing lens according to claim 12, wherein when the focal lengths of said third lens unit and said last lens unit are f3 and f4, respectively, following condition is satisfied:   0.05<|f3/f4 |<0.9.     
     
     
       14. A magnification changing lens according to claim 13, wherein when the focal length of the entire system at the wide angle end is fw, the following condition is satisfied:   4.0<|f4/fw |<15.0.     
     
     
       15. A magnification changing lens according to claim 1, wherein during the magnification change from the wide angle end to the telephoto end, each lens unit is moved so that the lateral magnifications of said second, third and last lens units may always increase. 
     
     
       16. A magnification changing lens according to claim 15, wherein when an amount of variation in the focal length of the entire system resulting from the magnification change from the wide angle end to the telephoto end is Δf and amounts of movement of said second lens unit and said last lens unit are M2 and M4, respectively, following conditions are satisfied:   0.3<|M2/Δf|<0.6       0.15<|M4/Δf|<0.6.     
     
     
       17. A magnification changing lens comprising in succession from an object side: a first lens unit having a negative refractive power;   a second lens unit having a positive refractive power;   a third lens unit having a negative refractive power; and   the last lens unit;   wherein a magnification change from wide angle end to telephoto end is accomplished by moving at least said second lens unit and said third lens unit toward the object side, and when the shortest distance of a back focus in the whole magnification change range is bf·min and focal length of the entire system at the wide angle end is fw, following condition is satisfied:   0.13<bf·min/fw<0.7.       
     
     
       18. A magnification changing lens according to claim 17, wherein said last lens unit has a positive refractive power. 
     
     
       19. A magnification changing lens according to claim 18, wherein when an amount of movement of said third lens unit and said last lens unit during the magnification change from the wide angle end to the telephoto end are M3 and M4, respectively, and an amount of movement toward imaging plane side is positive, following condition is satisfied:   -1<M4/M3<0.8.     
     
     
       20. A magnification changing lens according to claim 19, wherein the focal lengths of said second and third lens units are f2 and f3, respectively, following condition is satisfied:   0.8<|f3/f2|<1.4.     
     
     
       21. A magnification changing lens according to claim 20, wherein when the focal length of said last lens unit is f4 and the focal length of the entire system at the wide angle end is fw, following condition is satisfied:   1.5<f4/fw<7.0.     
     
     
       22. A magnification changing lens according to claim 17, wherein said last lens unit has a negative refractive power. 
     
     
       23. A magnification changing lens according to claim 22, wherein said last lens unit always has a magnification of one-to-one or higher. 
     
     
       24. A magnification changing lens according to claim 23, wherein when an amount of movement of said third lens unit and said last lens unit during the magnification change from the wide angle end to the telephoto end are M3 and M4, respectively, following condition is satisfied:   0≦M4/M3<3.     
     
     
       25. A magnification changing lens according to claim 24, wherein said last lens unit has at least one negative lens having its concave surface facing the object side. 
     
     
       26. A magnification changing lens according to claim 25, wherein when the focal lengths of said third lens unit and said last lens unit are f3 and f4, respectively, following condition is satisfied:   0.05<|f3/f4 |<0.9.     
     
     
       27. A magnification changing lens according to claim 17, wherein during the magnification change from the wide angle end to the telephoto end, each lens unit is moved so that lateral magnifications of said second, third and last lens units may always increase. 
     
     
       28. A magnification changing lens according to claim 27, wherein when the amount of variation in the focal length of the entire system resulting from the magnification change from the wide angle end to the telephoto end is Δf and the amounts of movement of said second lens unit and said last lens unit are M2 and M4, respectively, following conditions are satisfied:   0.3<|M2/Δf|<0.6       0.15<|M4/Δf|<0.6.